Press forming cold rolled steel sheet and a producing method thereof

ABSTRACT

A cold rolled steel sheet having excellent press formability and ageing properties for auto bodies, especially improved stretchability and slow ageing properties, and a method for preparation thereof by means of a continuous annealing method.

United States Patent. 1,1 1. Q

Gondo et a1. 5

1451 Nov." 18, 1975 PRESS FORMING COLD ROLLED STEEL [58] Field of Search148/ 12.1, 12.3, 12 SHEET AND A PRODUCING METHOD THEREOF I [56] 7References Cited [75] Inventors: Hisashi Gondo; Hiroshi Takechi; UNITEDSTATES PATENTS M1tun0bu all of Kisarazu; 3,607,456 9/1971 P616116, Jr.148/l2.l Norlmasa Uehara; Kunihiko 3,666,569 5/1972 Hultgren .1 148/12 cKomiya, both Of Kimitsu, all of 3,765,874 10/1973 Elias et a1. 148/12 CJapan 3,798,076 3/1974 Shimizu et a]. 148/12 C I 3,806,376 4/1974 T d tl l48/12.3 [73] Ass1gnee: Nlppon Steel Corporatron, Tokyo, O a a JapanPrimary ExaminerW. Stallard [22] Filed; Sept, 21, 1973 Attorney, Agent,or FirmToren, McGeady & Stanger 21 A1.N.:39, 1 1 pp 0 9 [57] ABSTRACT Acold rolled steel sheet having excellent press formv [30] ForelgnApphcat'on Pnomy Data ability and ageing properties for auto bodies,espe- Sept. 26, l972 Japan 47-95728 improved tretchability and lowageing proper- Sept. 26, 1972 Japan 47-95729 fi and a method forpreparation thereof by means of a continuous annealing method. [52] US.Cl. 148/12 C; 148/123 51 1111. C1? C21D 9/48 1 Clam" 5 Drawmg FlguresIl-Z I10 8 \1 10:5 s E I06 4 /0.4- g .1 o I02 l E /0.0

0VEZ4 65/46 COIYOITID YG U.S. Patent Nov. 18, 1975 Sheet 2 of2 3,920,487

4Z mmm 01/524 6.97/76 6'0/Y0/T/0 Y5 FIG. 2

TIME (/w/vu re) FIG. 3

PRESS FORMING COLD ROLLED STEEL SHEET AND A PRODUCING METHOD THEREOFConventionally, press forming cold rolled steel sheets 7 forapplications such as auto bodies have been required to have excellentdeep drawability and stretchability and as well as good ageing property.

In the deep drawing operation, only a small force for preventingwrinkles is given to the steel sheet being press formed, and said sheetis formed from its outer portions into the dies by the punch beingpressed thereon. Therefore, the most important property required for thepress forming steel sheet is a high 7 value. On the other hand, in thestretching operation, the wrinkle preventive force is remarkablyincreased, and only a slight portion of the steel sheets are pressedinto the dies and only portions contacting the punch are expanded andpressed. In this case, the most important characteristic is a highErichsen value, and additionally it is desired that yield stress is lowboth for deep drawability and stretchability, and elongation is large.

Cold rolled steel sheets subjected to the press forming are generallyannealed after the cold rolling, but the steel sheets as annealed havehigh yield point elongation so that when the sheets are press formed,stretcher strain is caused and the surface appearance is damaged. Forthis reason, the sheets after the annealing are subjected to a temperrolling to eliminate the yield point elongation.

Even in this case, however, if the sheet is left for a long timethereafter, before the subsequent press forming, the yield pointelongation is restored due to the ageing of carbon and nitrogen in solidsolution and thus the stretcher strain is again caused during the pressforming.

The cold rolled steel sheet in which the yield point elongation is notrestored even when the sheet is left for a long time after the temperrolling, are called non-ageing steel sheets and the steel sheets inwhich the restoration of the yield point elongation is slow, are calleda slow ageing steel sheets. And the steel sheets for press forming havenon-ageing, or at least a slow ageing property, deep drawability andstretchability.

Conventionally, the cold rolled non-ageing or slow ageing steel sheetshaving above mentioned properties for auto bodies have been commerciallyproduced by applying a box annealing to Al-killed steel.

The Al-killed steel contains aluminium in such an amount as to be enoughto form AlN during the heating of the annealing and is effective toimprove the 7value in the cource of the AlN formation, and fixes thesolid solution nitrogen as AlN, thus improving the ageing property.However, in case of the Al-killed steel, pipes are formed during theingot making in the head portion of the ingot, which cause inner defectsin the final product, and thus it is necessary to cut off the headportions.

annealing requires normally more than 60 hours and thus is not desirablefor the production efficiency. In

'As the Al-killed result, the production yield of the Alkilled steel isremarkably lowered. In addition, the box 2 killed steel of lowproduction yield and applying the box annealing of low productionefficiency.

Therefore, one of the object of the present invention is to provide acold rolled steel sheet having an excellent press formability and ageingproperties for auto bodies, especially improved stretchability and slowageing properties.

Another object of the present invention is to provide a method forproducing the cold rolled steel sheet having an excellent pressformability and ageing property for auto bodies, especially improvedstretchability and slow ageing property, by means of a continuousannealing method.

The purposes of the present invention are:

l. a cold rolled steel sheet for press forming having a chemicalcomposition comprising not more than 0.1% carbon and not more than 0.30%manganese in which 0.002. (ppm) -0.03 s K s 0.17 +0.002N(ppm) where,

2. a method for producing a press forming cold rolled steel sheetcomprising not more than 0.1% C and not more than 0.30% Mn andsatisfying the conditions 0.002 X N ppm 0.03 s K S 0.17 +0.002 X N ppwhere which comprising an ordinary hot rolling step, a cold rolling stepand a continuous annealing step;

3. a method according to above (2), which further comprises anoverageing step;

4. a method according to above (3), in which the annealing done at atemperature between the recrystallization temperature and 900C for notlonger than 5 minutes, and the overageing is done between 300 and 500Cfor not longer than 10 minutes;

5. a method for producing cold rolled steel sheet having good pressformability, comprising passing a cold rolled steel strip containing notmore than 0.1% carbon through a continuous annealing furnace composed ofa heating and a soaking zone, a primary cooling zone, an overageingzone, and a secondary cooling zone arranged in series, in which 30seconds to 5 minutes heating and soaking is effected at a temperaturebetween 650 and 900C in the heating and soaking zone, cooling down tothe starting temperature of overageing or below is effected in theprimary cooling zone, and overageing with starting temperature between400 and 500C and then stepwisely or continuously lowering temperaturebetween 200 and 500C is effected for 2 to 10 minutes and the overageingis completed between 200 and 350C; and

6. a method for producing a slow ageing cold rolled steel plate forpress forming comprising hot rolling a steel slab having a chemicalcomposition comprising not more than 0.1% carbon and not more than 0.30%manganese, in which 0.002.N(PPm) 0.002.N(ppm) where,

55 55 K Mn(%) 51%) coiling the hot rolled strip at a temperature between600 and [850 2.1 X 10 X C(%)]C, cold rolling the hot rolled strip by aconventional method, and passing cold rolled strip thus obtainedcontinuously through a continuous annealing furnace composed of aheating and soaking zone, a primary cooling zone, an overageing zone,and a secondary cooling zone arranged in series, in which 30 seconds tominutes heating and soaking is effected at a temperature between 650 and[I680 4.6 X X C(%) the hot coiling temperature (C)] C in the heating andsoaking zone, cooling down to the starting temperature of overageing orbelow is effected in the primary cooling zone, and overageing withstarting temperature between 400 and 500C and then step-wisely orcontinuously the lowering temperature at the starting temperature ofoverageing between 200 and 500C is effected for 2 to 10 minutes in theoverageing zone, the overageing is completed between 200 and 350C.

The present invention will be described in detail referring to theattached drawings.

FIG. 1 a, b and c respectively show a graph of heat cycles of isothermaloverageing and stepwise overageing.

FIG. 2 is a graph showing Erichsen values of steel materials subjectedto isothermal overageing and stepwise overageing.

FIG. 3 is a graph showing a heat cycle in the continuous annealingfurnace.

In general, metal materials having a larger grain size are softer andmore suitable for cold working and show larger Erichsen values,elongation, and T values and lower yield stress with good ageingproperties. The size of the grains depend on the chemical compositionand the heating history in the production process.

Mn, S and O are elements which harden the steel and lower deepdrawability and stretchability by their solid solution hardening effectsorv by grain refinement due to their recrystallization preventingeffects or by both. However, if Mn combines with S and O to form MnS andMnO, the grain refinement-tendency due to the solid solution hardeningand the recrystallization preventing effects disappear and large grainsare obtained, thus improving deep drawability and stretchability. Inthis case, however, if Mn or any or both of S and 0 remains excessivelyin a large amount, they are harmful. In case the coefficient K isestablished as under Atomic wei ht of Mn K M"(%) Atomic weight of SAtomic weight of Mn 7 Atomic weight of O 0( a) 55 55 Mn(%) 55170) T0070)if K is remarkably smaller than zero, this means that either or both ofS and O is excessive in a large amount, and if K is remarkably largerthan zero, this means that Mn is excessive in a large amount.

Therefore, deep drawability and stretchability can be improved bysetting the K value in a certain range. As mentioned above, the largeexcessive amount of Mn is harmful], but a small excessive amount of Mnis very effective to improve the ageing property. If MnS and MnO arepresent in alarge amount, they by themselves cause fractures during thepress forming operation, and it is necessary to maintain MnS and MnO aslow as possible. Regarding the carbon content, more than 0.1% carboncauses material hardening, and thus the carbon content should be notmore than 0.1%.

For the reasons set above, the chemical composition of the slow ageingcold rolled steel sheet for press forming according to the presentinvention is limited so as to satisfy the following conditions.

Carbon 5 0.1%

Manganese s 0.30%, preferably 5 0.25%

55 55 K Mn(%) S(%) T0070) and 0.002.N (ppm) 0.03 s" K s 0.12 0.002.N

In this case, a low level of N content is preferable, especially Ncontent less than 25 ppm improves some mechanical properties such asdrawability and ageing property.

By balancing the four components, Mn, S, O and N appropriately accordingto the present invention, or additionally by restricting the upper andthe lower limits of the hot coiling temperature, it is possible toproduce a cold rolled steel plate suitable for press forming of the autobodies and comparable with steel sheet produced by box annealing method.

Next, the second purpose of the present invention will be describedhereinunder.

The cold rolled steel sheet to which the method of the present inventionis directed, may be produced from any of a rimmed steel, a capped steel,a killed steel and other steels containing various alloying elements andimpurities, so far as the carbon content is not more than 0.1% and thedesired results of the present invention can be obtained.

The feature of the present invention lies in the uses of the continuousannealing method as specified.

According to the box annealing method, steel coils or cut sheets areheated for soaking and then cooled in a piled condition and the heatcapacity of the piled steel coils or cut plates is so large that ittakes about 10 hours to heat the whole of the piled coils or cut platesto a predetermined annealing temperature, and the cooling after thesoaking is very slow and it usually takes more than one day to cool thecoil or the plate to a room temperature from the soaking temperature.

Whereas, according to the continuous annealing method of the presentinvention, the steel strip is once uncoiled into a strand form andsupplied to an annealing furnace so that the heat capacity of thematerial to be annealed is so small that it is possible to heat thematerial to the soaking temperature in a short time, that isrecrystallization can be completed by 30 seconds to 5 minutes soaking ata temperature between a recrystallization temperature and 900C, withsimilar results as in the box annealing method. When the soakingtemperature is lower than recrystallization temperature or the soakingtime is shorter than 30 seconds, the recrystallization is not enough,while when the soaking temperature is higher than 900C and the soakingtime is longer than 5 minutes, no substantial improvement of thematerial is obtained with only lowering of the production efficiency andalso 7 value remarkably decreases.

As the cooling rate after the soaking is very slow in the box annealing,the carbon atoms in solid solution present at the beginning of thecooling gradually pre- 1 ;cipitate as carbides duringthe cooling incorresponproving the stretchability and the ageing property of thematerial.

According to the continuous annealing, as the heat capacity of thematerial passing through the heating and soaking zone of the furnace isso small that the material is rapidly cooled down to the roomtemperature within minutes. Therefore, a considerable part of the solidsolution carbon which was present in the material at the beginning ofthe cooling is retained in the solid solution and thus thestretchability and the ageing property of the material are deterioratedby the solid solution hardening. In order to prevent the problem, thematerial is cooled down to the overageing temperature or below and thenthe material is subjected to an overageing treatment at a temperaturebelow the soaking temperature to precipitate the excessive solidsolution carbon as carbides.

When the overageing temperature is high, the diffusion rate of thecarbon is rapid and thus the carbide precipitation is completed in ashort time, but the solid solution limit of carbon is so high that theamount of the residual solid solution carbon is large. Therefore, theoverageing is not complete. On the other hand, when the overageingtemperature is low, the diffusion rate of carbon is so slow that a longtime is required for the completion of the carbide precipitation, butthe solid solution limit of carbon is so small that the,

amount of the residual solid solution carbon is very small and thus theoverageing is complete. In this way the selection of the overageingtemperature has an alternative significance that it increases theproduction effeciency or assures the complete material quality such asstretchability. In the conventional art, either of the advantages hasbeen sacrificed.

According to the present invention, the material is cooled from thesoaking temperature to the starting temperature of the overageing orbelow, and overageing with starting temperature between 400 and 500C iseffected for 2 to 10 minutes with the temperature stepwisely orcontinuously lowering between 200 and 500C and the overageing iscompleted between 200 .and 350C, thereby the advantages that the carbideprecipitation is completed rapidly by a high temperature overageing andthat the residual solid solution carbon is remarkably reduced by a lowtemperature overageing are assured while the defects of the high and lowtemperature overageing are eliminated, thus maintaining thestretchability, preventing the ageing due to the solid solution carbon,and improving the productivity.

For the practice of the present invention, it is desirable that thecooling from the soaking temperature to a temperature below the startingtemperature of the overageing is conducted at a rate of 5 to C/second.Even during the cooling from the soaking temperature, the diffusion rateof the carbon is remarkably high immediately below the soakingtemperature, it is not desirable that the cooling rate in the primarycooling zone exceedes 30C/second in order to utilize also the carbideprecipitate in this stage. Also with a cooling rate more than30C/second, the over saturation degree of special advantage is notobtained, and the productivity is lowered. The cooling rate of 5 to30C/second from the soaking temperature to a temperature below theoverageing temperature is a preferable range and not an essential range.The desired results of the present invention can be obtained even whenthe cooling rate is outside the above range.

Whenthe starting temperature of the overageing is higher than 500C, thesolid solution of carbon becomes relatively large and the overageing ismeaningless. At the beginning of the overageing between 400 and 500C,carbides are formed at the grain boundaries when the above primarycooling rate is applied. For example, in case of the material containing0.02% carbon, the solid solution limit at 700 and 400C is respectively0.02% and 0.0036% and thus most of the solid solution carbonprecipitates as grain boundary carbides at this stage, but a smallamount of solid solution carbide remains. The residual solid solutioncarbon even in such a small amount is harmfull to the stretchability andthe ageing property. Just before the finishing of'the overageing between200 and 350C, the residual solid solution carbon precipitates as finecarbides in the grains and the residual solid solution carbon is reducedto a negligible amount, for example to about 0.004% which is asolubility limit for carbon at without damaging the stretchability andthis can not be expected in the box annealing and one of the features ofthe present invention. When all of the solid solution carbon which waspresent at the time of the soaking precipitates as the fine carbide inthe grains, the precipitation hardening as mentioned before is causedand thus the stretchability is deteriorated, although advantageous forthe slow ageing.

According to the present invention, 2 to 10 minutes are given from thebeginning of the overageing (400 to 500C) to the completion of theoverageing (200 to 350C) during which the overageing temperature islowered stepwisely or continuously. With less than 2 minutes ofoverageing, the object of the overageing is not attained, while withmore than 10 minutes of overageing, the advantage of the continuousannealing in the improved productivity is not attained. The most desiredconditions of the overageing are: the beginning of the overageingbetween 400 and 450C, the completion of the same between 250 and 300C,and the total period of the overageing of 3 to 6 minutes.

The overageing with the continuously lowering temperature used hereinmeans the case where the temper ature lowering rate at each position inthe overageing zone is within i 30% of the average temperature loweringrate obtained by dividing the difference between the beginningtemperature and the completing temperature of the overageing by thetotal overageing time. And the overageing with the stepwisely loweringtemperatures and used herein means as under. The overageing zone isdivided into two or more isothermal overageing zones, where theoverageing temperature stepwisely lowers, each of the isothermal timesis maintained within i 30% of the average isothermal overageing timeobtained by equally dividing the total passing time in the totalisothermal zones by the number of the isothermal overageing zones andthe temperature range in each of the isothermal overageings ismaintained within i 30% of the average overageing temperature differenceobtained by dividing the difference between the beginning temperatureand the completing temperature of the overageing by the number of theisothermal overageing zones and the transition zone between theisothermal overageing zones is passed through in time shorter than 50%of average isothermal overageing time.

When the above overageing conditions are not applied, and only theisothermal overageing between 400 and 500C is applied, a harmful amountof residual solid solution carbon for the stretchability and the ageingproperties will remain even if the long isothermal overageing may beapplied, because the solid solution limit of carbon in the abovetemperature range is high, and when only the isothermal overageingbetween 200 and 350C is applied, the precipitation of solid solutioncarbon is so slow that a long time is required by the overageing, thusno practical value is assured. And particularly, application of only theisothermal overageing between 200 and 300C is not desirable because alarge amount of solid solution carbon is converted into fine carbides inthe grains and the stretchability is deteriorated by the precipitationhardening. Overageing conditions mentioned above are preferableoperation.

In the secondary cooling zone, it is desirable to cool the material downto near the room temperature as rapid as possible for improvement of theproductionefficiency.

As described above, it has been made possible by the present inventionto produce by a continuous annealing method, a cold rolled steel sheethaving stretchability comparable to that obtained by the box annealingat high production efficiency, and the commercial advantages of thepresent invention are very remarkable.

Next, the purpose of the present invention will be describedhereinbelow.

The slow ageing cold rolled steel sheets for press forming according toa purpose of the present invention will have more desirable propertiesby applying the continuous annealing method as described hereinbefore.In this case, however, the hot coiling temperature is important.

First, a hot rolled steel strip is prepared from a rimmed capped orkilled steel having the following chemical compositions;

Mn 0.30% preferably 4 0.25%

0.002.N (ppm) 0.03 s K s 0.17 0.002.N

For the production of the above hot rolled steel strip, the hot coilingtemperature is limited as under in order to promote the reactions of MnS -r MnS and Mn MnO for the reasons mentioned hereinbefore. When thestrip is coiled after the hot rolling, the cooling rate after the hotrolling is very slow, so that the strip is maintained at the hightemperature for a long time, and particularly when the coiling is doneat a temperature not lower than 600C, more preferably not lower than650C, the formation of MnS and MnO by the above reaction is remarkablypromoted. On the other hand when the coiling temperature is too high,coarse carbides are formed, which promote the fracture during the pressforming. Thus the upper limit of the coiling temperature is limited to[850 2.1 X 10 x C(%)]C.

The hot rolled steel strip thus obtained is cold rolled and thencontinuously annealed by passing the strip through the continuousannealing furnace in which a heating and soaking zone, a primary coolingzone, an overageing zone and a secondary cooling zone, are arranged inseries.

In order to obtain satisfactorily large grain size in the heating andsoaking zone, soaking of not shorter than 30 seconds at a temperaturenor lower than recrystallization temperature is required. On the otherhand, even if the soaking is conducted for a longer time than 5 minutesno special advantage is obtained for the enlargement of the grains, butthe longer time of the soaking only lowers the production efficiency.Thus the soaking time should be limited to not more than 5 minutes, whenthe annealing temperature is too high, coarse carbides are formed, whichpromote the fracture during press forming. Thus the upper limit oftemperature should be determined in relation to the hot coilingtemperature. In this way, the soaking condition of the present inventionis limited as between 650C and [1680 4.6 X 10 X C(%) the hot coilingtemperature (C)]C for 30 seconds to 5 minutes.

Further, the secondary cooling should be done as rapidly as possible to40C. This is desirable for the improvement of the production efficiency.The steel strip cooled down below 40C which has been passed through thesecondary cooling zone is, if necessary, subjected continuously 1.0 to1.5% temper rolling to eliminate the yield point elongation and toprevent the stretcher strain and subjected to levelling for shapecorrection, and then coated with rust preventive oil or solidlubricating oil for the press forming, finally coiled for shipment.

The present invention will be more clear from the following examples.

EXAMPLE 1 A capped steel refined in a convertor and comprising 0.04% C,0.23% Mn, 0.010% S, 0.011% P, 0.01% Si, 0.042% 0 and 15 ppm N, with thebalance being iron and unavoidable impurities was made into slabs by anordinary method. After soaking at 1200C for 5 hours, the slab was hotrolled into 2.6 mm thickness with a finishing temperature of 890C andcoiled at 710C. After acid pickling for scale removal, the hot rolledstrip was cold rolled into 0.8 mm thickness and electrically cleaned,and subjected to a recrystallization annealing at 700C for 2 minutes ina salt bath furnace. Then an isothermal overageing and a stepwiseoverageing as shown in FIG. 1 were conducted for 5 minutes of overageingusing salt baths maintained at 600, 500, 400 and 300C, an oil bathmaintained at 200C. The cooling rate from the annealing temperature tothe starting temperature of the overageing was 10 to 15C/second.

Erichsen values of the samples are shown in FIG. 2, and the samples G, Hand J, which are within the scope of the present invention showparticularly high Erichsen values, which are similar to that of thesample X which was box annealed for 4 hours at 700C for comparison.

EXAMPLE 2 Three rimmed steel ingots refined in a convertor andcomprising 0.04% C, 0.22% to 0.24% Mn, 0.01% Si, 0.008% to 0.012% S,0.010 to 0.012% P, 0.032 to 0.041% 0, 9 to 12 ppm N, with the balancebeing iron and unavoidable impurities were processed in the same way asin Example 1 up to the cold rolling, passed through a verticalcontinuous annealing furnace provided with an electrical cleaning devicein the front and a temper leveller and an oil coater in the rear, whereannealing cycles (a), (b) and (c) shown in FIG. 3 were applied and 1.5%temper rolling was conducted. The properties of the products thusobtained are shown in Table 1 together with the properties of theproduct obtained by a box annealing of a coil of the above compositionat 700C for 4 hours and 1.5% temper rolling.

Table 1 Continuous Annealing Cycles and Material Properties AnnealingCycle (a) (b) (c) Box Annealing Yield Stress (kg/mm 18.8 21.1 26.5 19.2Tensile Strength (kg/mm) 32.1 32.3 35.6 32.4 Fracture Elongation 47.042.6 38.1 46.0 Erichsen Value (mm) 10.9 9.7 9.2 10.9 Hardness (HRB) 4044 53 41 In case of the annealing cycle (a) according to the presentinvention, with only 3 minutes of overageing, products completelycomparable with that of the box annealing was obtained.

EXAMPLE 3 Cold rolled steel strip of 0.8 mm thickness obtained from hotrolled steel strip of 2.6 mm thickness coiled at 680C, having a chemicalcomposition of 0.04% C, 0.015% Mn, 0.01% Si, S, 0.013% P, 0.040% 0 and l15 ppm N with the balance being iron and unavoidable 1.5% temperrolling.

The results of the tests of the mechanical properties of the samples areshown in Table 2, from which it is clear that in case of the primarycooling rates of 10 C/second and C/second which are outside the presentinventiom'the Erichsen value is low and the stretchability is also poor.

Table 2 Primary Cooling Rates and Material Properties Primary CoolingCapped steels No. l to No. 7 and Al-killed steel No. 8 refined in aconverter and having the chemical com- 7 positions shown in Table 3 werehot rolled with a finishing temperature between 890 and 900C into hotrolled strips of 2.6 mm thickness, coiled at various coilingtemperatures shown in Table 3, acid pickled, cold rolled into coldrolled strips of 0.8 mm thickness. The

steels No. l to No. 6 were heated to 700C in one minute and 10 secondsin the continuous annealing furnace, maintained at 700C for one minute,cooled to the starting temperature of the overageing of 450C with acooling rate of 15C/second, subjected to a stepwise overageing composedof three isothermal overageings of 450C for one minute, 350C for oneminute and 250C for one minute, with the transition times between thefirst and the second isothermal overageings and between the second andthe third isothermal overageings being 20 seconds, cooled to the roomtemperature in one minutes and 20 seconds, and subjected to 1.3% temperrolling.

For comparison, the capped steel No. 7, and the Alkilled steel No. 8were box annealed at 700C for 4 hours and 16 hours respectively and weresubjected to 1.5% temper rolling. The results of tests on the mechanicalproperties for these comparison samples are shown in Table 4. In thesteels No. l and No. 2 treated according to the present invention, theyield stress, the elongationthe r value and the Erichsen value are allon the same level to that of the box annealed Al-killed steel No. 8 andthe press formability is better than that of the steel No. 8. Althoughthe ageing property is somewhat lower than that of the Al-killed steelNo. 8, the yield point elongation after the ageing is remarkably smalleras compared with the box annealed capped steel No. 7 and a slow ageingproperty is completely obtained.

Table 3 Chemical Compositions & Conditions for Hot Coiling Etc.

ing tempe- Table Ii-continued Srtleel .Chemical Compositions C Si Mn" SP O N Sol.Al

K K K rature (C) E F Remarks 0.0l8 0.046 3 0.1 82 710 787 832 PresentInvention 0.008 0.041 0.208 680 766 816 0.012 0.029 0.212 680 766 816Outside Present Invention (too small K value) 0.008 0.214 0.208 680 766816 I (too large K value and Mn) 0.004 0.065 0.204 750 703 608 (too highcoiling temperature) 0.046 0.200 g 570 766 946 (too 1 low coilingtemperature) 0.000 0.002 0.163 0.198 570 766 946 Comparison Box annealedMaterial 550 745 900 K, 0.002 x N (PPm) 0.03 K,= 0.17 0.002 x N (ppm) IHot Coiling Temperature (C) Table 4 Mechanical Properties What isclaimed is: l. A method for producing a slow ageing cold rolled steelplate for press forming comprising hot rolling a steel slab having achemical composition comprising not more than 0.1% carbon and not morethan 0.30% manganese in which coiling the hot rolled strip at atemperature between 600 and [850 2.1 X 10 X C(%) ]C, cold rolling thehot rolled strip by a conventional method, and passing the cold rolledstrip thus obtained continuously through a continuous annealing furnacecomposed of a heating and soaking zone, a primary cooling zone, anoverageing zone, and a secondary cooling zone arranged in series, inwhich 30 seconds to 5 minutes heating and soaking is effected at atemperature between 650 and [1680 4.6 X 10 X C(%) the hot coilingtemperature (20 C) C in the heating and soaking zone, cooling down tothe starting temperature of overageing or below is effected in theprimary cooling zone, and overageing with a starting temperature between400 and 500C and then step-wisely or continuously lowering thetemperature at the starting temperature of overageing between 200 and500C is effected for 2 to 10 minutes in the overageing zone, and theoverageing is completed between 200 and 350C.

1. A METHOD FOR PRODUCING A SLOW AGEING COLD ROLLED STEEL PLATE FORPRESS FORMING COMPRISING HOT ROLLING A STEEL SLAB HAVING A CHEMICALCOMPOSITION COMPRISING NOT MORE THAN 0.1% CARBON AND NOT MORE THAN 0.30%MANGANESE IN WHICH 0.002.N (PPM)-0.03<K<0.17+0.002.N(PPM) WHERE.